Thermal Radiation from Carbon Nanotubes in the Terahertz Range

Abstract

The thermal radiation from an isolated finite-length carbon nanotube (CNT) is theoretically investigated both in near-and far-field zones. The formation of the discrete spectrum in metallic CNTs in the terahertz range is demonstrated due to the reflection of strongly slowed-down surfaceplasmon modes from CNT ends. The effect does not appear in semiconductor CNTs. The concept of CNT as a thermal nanoantenna is proposed.PACS numbers: 78.67. Ch, 44.40.+a Carbon nanotubes -quasi-one-dimensional carbon molecules -have found numerous real and potential applications as building blocks of nanoelectronic circuits [1] and nanoscale optical elements [2]. Among others, the idea of CNT-based optical devices enabling control and enhancement of radiation efficiency on the nanoscale, i.e. nanoscale antennas for infrared and visible light, is actively discussed [3,4,5,6]. Noise properties and operational limits of such devices are substantially determined by the thermal fluctuations of electromagnetic field. Fundamental interest to the thermal radiation is also dictated by the ability of nanostructures to change the photonic local density of states (LDOS), i.e. the electromagnetic vacuum energy [2,7,8]. The effect has been observed in microcavities, photonic crystals, nanoparticles in the vicinity of surface-plasmon resonances, etc. [2]. Thus, as the electromagnetic fluctuations are defined by the photonic LDOS, investigation of the thermal radiation is expected to bring new opportunities for the reconstruction of photonic LDOS in the presence of nanostructures. In turn, the photonic LDOS is a key physical factor defining a set of well-known quantum-electrodynamical effects: the Purcell effect [8], the Casimir-Lifshitz forces [9], the electromagnetic friction [2], etc. All aforesaid stimulates investigation of thermal radiation on nanoscale.During last decade, there has been considerable interest to optical properties of surface-plasmon structures [10,11]. In particular, spherical gold particles in the vicinity of the plasmon resonance have been shown to be an effective nanoantennas [12]. Recently, the strong coherent coupling between individual optical emitters and guided plasmon excitations has been predicted [13] introducing thus quantum optics of nanoplasmonic structures. Thermal radiation in systems with surface plasmons is influenced by the near-field effects and is known to be considerably different from the black-body radiation [14,15]. Earlier theoretical studies of CNTs showed the existence of low-frequency plasmon branches [16,17] and the formation in CNTs of strongly slowed-down electromagnetic surface waves [18]. Such waves define pronounced Purcell effect in CNTs [19] and potentiality of CNTs as Cherenkov-type emitters [20]. Geometrical resonances -standing surface waves excited due to the strong reflection from the tips of finite-length CNTs -qualitatively distinguish CNTs from the planar structures investigated in [14,15]. One can expect an essential role of these resonances in the formation of the CNT's therm...

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